Deprecate inject decorator

[rmk135]

Usage of @inject decorator can lead to bad design and could be considered as anti-pattern.

• Promotion of usage of @inject decorator should be terminated (README, Docs).
• All public examples should be redesigned / rewritten to avoid usage of @inject decorator.
• Documentation page about @inject decorator should be left "as is", but:
  • There should be no links to it from latest version of docs.
  • There should be a warning about deprecation of functionality in the top of the page.
• Access / usage of @inject decorator should produce a DeprecationWarning.
• Change should be backward compatible with all 2.x.x releases.

[coveralls]

Coverage remained the same at 100.0% when pulling 38c9d2f on deprecate_inject_decorator into d9f4aed on master.